I want to present 4 figures inside one float, so I'm using the following format:

Although, when I present this float I get:

As you can see, there is a small vertical space between the first "line" to the other. I want the 4 figure to be aligned and symmetric.
Appreciate your help, thanks!
Note: the lyx file + latexpdf + pdf available in the comments.
Latex code:
% Preview source code for paragraph 0
\begin{figure}
\begin{centering}
\subfloat[]{\includegraphics[width=0.45\columnwidth]{../Documents/MEGA/Technion/Msc/Thesis/Code/code_files/Thesis/images/edges_bias/original_PH}
}\hspace{1px}\subfloat[]{\includegraphics[width=0.45\columnwidth]{../Documents/MEGA/Technion/Msc/Thesis/Code/code_files/Thesis/images/edges_bias/composed_PH}}\\\subfloat[]{\includegraphics[width=0.45\columnwidth]{../Documents/MEGA/Technion/Msc/Thesis/Code/code_files/Thesis/images/edges_bias/original_PH}
}\hspace{1px}\subfloat[]{\includegraphics[width=0.45\columnwidth]{../Documents/MEGA/Technion/Msc/Thesis/Code/code_files/Thesis/images/edges_bias/composed_PH}}
\par\end{centering}
\centering{}\caption{Description: (a) (b) (c) (d)}
\label{fig:filtering_process}
\end{figure}


\newlinein the screenshot. It is even in the title. – Johannes_B Jan 15 '17 at 10:40